A fundamental requirement for successful laser cutting is that the exiting laser beam is sufficiently well centered to the orifice of a gas nozzle of a laser cutting head. If that is not the case, the cutting performance will not be uniform with respect to different cutting directions. Cut surface can get rough and in extreme cases the gas nozzle can get heated up to the point of melting. Reasons for misalignment can be poor mechanical tolerance of gas nozzles, improper alignment of focus lens (or collimator lens), a general alignment problem in the machine, e.g. improper alignment of the different mirrors for CO2-laser, effects from collisions between the laser cutting head and a workpiece.
By far the most common method is to use a series of beam shots on an adhesive tape target fixed to the gas or cutting nozzle. This requires the use of the actual laser beam at low power and short exposure (100 ms region). The contour of the nozzle orifice is visible as well as the resulting burn hole. This is an iterative process of repeatedly adjusting the laser beam and measuring the position of the burn hole relative to the nozzle orifice. Such process can take 5 to 10 minutes to complete depending on the skill level of the operator. This is usually followed by a cutting test observing the behavior of the blast of molten particles below the plate or sheet processed. However, the tape method poses a safety threat as it involves the use of a real laser beam.
In addition to the less sophisticated “tape method” described above there are a couple of attempts to use camera technology and fixed measuring stations.
DE 10 2004 005 902 B3 discloses an alignment unit to be used with a laser cutting head for adjusting the laser beam relative to an orifice of a cutting nozzle of the laser cutting head. The alignment unit comprises a housing in which a centering ring with an opening is provided for receiving the cutting nozzle when the laser cutting head is in a test position. Behind the centering ring an infrared conversion screen and a camera module are arranged in the beam path of the laser beam. To avoid damage to the alignment unit and/or the cutting nozzle when the laser cutting is brought into its test position the under-side of the housing of the alignment unit is provided with an elastic damping material.
When the laser cutting head is in the test position, i.e. the cutting nozzle rests on the centering ring, the laser beam is projected onto the infrared conversion screen. An image of the screen showing the current position of the laser beam is captured by the camera module and transmitted to a display.
EP 1 967 316 A1 discloses another system for determining a relative position of a nozzle of a laser machining head with respect to a laser beam axis. The system comprises a unit for lighting a part of the nozzle, a camera unit for recording an image of an orifice of the nozzle, and a unit for displaying the images acquired before and/or after processing. The laser machining head needs to be positioned with respect to the camera unit. Then, camera pictures are recorded before and after activation of a pilot laser. To determine the position of the laser beam relative to the opening of the nozzle image processing is performed.
None of the above solutions are considered portable, but each requires a fixed measuring position in the work envelope of the laser processing machine. This is a definite disadvantage, in particular when using CO2-lasers with mirror beam delivery systems. As a consequence alignment or misalignment of the laser beam relative to the nozzle opening cannot be measured in more than one position.
Further, a dedicated system for determining alignment or misalignment of the laser beam per laser processing machine can hardly be justified from a cost perspective.